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Characterizing ice particles using two-dimensional reflections of a lidar beam

M. Goerke, Z. Ulanowski, G. Ritter, E. Hesse, R. R. Neely, L. Taylor, R. A. Stillwell, and P. H. Kaye

Open Access Open Access

Abstract

We report a phenomenon manifesting itself as brief flashes of light on the snow’s surface near a lidar beam. The flashes are imaged and interpreted as specular reflection patterns from individual ice particles. Such patterns have a two-dimensional structure and are similar to those previously observed in forward scattering. Patterns are easiest to capture from particles with well-defined horizontal facets, such as near-horizontally aligned plates. The patterns and their position can be used to determine properties such as ice particle shape, size, roughness, alignment, and altitude. Data obtained at Summit in Greenland show the presence of regular hexagonal and scalene plates, columns, and rounded plates of various sizes, among others.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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Supplementary Material (1)

NameDescription
Visualization 1: MP4 (2218 KB)      Time-lapse video of 2500 images photographed at Summit on the 6th December 2016 from 1851 to 2110 UTC

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Figures (14)
Fig. 1.
Fig. 1. Scattering from a lidar beam onto snow: 30 superimposed images, 1.3 s exposure time each. Recorded on 6 December 2016. The lidar hut is located behind and to the right of the camera, as shown in Fig. 2.

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Fig. 2.
Fig. 2. Cartesian coordinate system for locating the 2-D patterns. The snow surface is on the X Y ( Z = 0 ) plane, and the lidar beam is on the Y Z ( X = 0 ) plane. Point A is the location of a specular reflection for a horizontal ice plate, B for a plate tilted about the Y -axis, C the location of the camera ( X = − 3 , Y = 4.5 , Z = 1.5   m ), D the lidar aperture ( X = 0 , Y = 2.5 , Z = 4   m ), and E the footprint of the lidar hut.

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Fig. 3.
Fig. 3. Representative selection of individual patterns observed at Summit Station on 6 December 2016. The patterns were stretched anamorphically to correct for the viewing angle. The mean width of the visually discernible parts of the patterns was 1 m.

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Fig. 4.
Fig. 4. Positions of 2-D patterns recorded on 6 December 2016 and analyzed to recover ice particle properties. The camera, lidar aperture, and roof shadow for the horizontal crystals (dashed line, see text) are also shown. The coordinates are as in Fig. 2.

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Fig. 5.
Fig. 5. Profile of temperature (red), dew point (green), relative humidity with respect to liquid water (pink), and ice (black) from a radiosonde launched at 2315 UTC on 6 December 2016. The inset shows the lowest 500 m. The humidity uncertainty is 4%.

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Fig. 6.
Fig. 6. CAPABL lidar and MMCR radar data from 1200 to 2400 UTC on 6 December 2016 at Summit; the camera imaging period is highlighted. The top three panels show the backscatter ratio (i.e., the ratio of total scattering to molecular scattering), depolarization, and diattenuation observed by CAPABL [17]. The middle panel is the cloud mask derived from CAPABL [18]. The bottom three panels show the reflectivity, Doppler fallspeed (where a positive value is downward), and the spectrum width (representative of turbulence or differential fall velocity). Since radar and lidar are complementary sensors that experience differing amounts of attenuation due to scatter by the cloud, CAPABL rarely observes the cloud top ( ∼ 5   km ), while the radar is able to penetrate the entire layer.

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Fig. 7.
Fig. 7. Left: pattern photographed at Summit, image width 5°, stretched anamorphically to compensate for the viewing angle. The pattern angular scale is approximate, based on particle altitude estimate. The observed image was stretched in the vertical direction to correct for the viewing angle. The particle diameter estimated from fringe separation was 150 μm. Right: theoretical 2-D scattering pattern from a 50 μm diameter, 1 μm thick, hexagonal plate tilted at 32° w.r.t. the incident direction. The pattern is centered on the direction of the specular reflection from the basal facet and is 10° wide.

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Fig. 8.
Fig. 8. Left: as in Fig. 7, but the size is 130 μm. Right: theoretical pattern from a 50 μm diameter, 4 μm thick, slightly rounded hexagonal plate tilted at 32° w.r.t. the incident direction, image width 10°.

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Fig. 9.
Fig. 9. Left: as in Fig. 7, but the size is 120 μm. Right: theoretical pattern for a strongly rounded hexagonal plate 50 μm diameter, 1 μm mean thickness, incident angle 20°, image width 20°.

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Fig. 10.
Fig. 10. Left: as in Fig. 7, but the size is 175 μm. Right: theoretical pattern for a 40 μm diameter, nearly spheroidal plate with 0.5 μm mean thickness, incident angle 20°, image width 20°.

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Fig. 11.
Fig. 11. (a)–(d): side-on and perspective views of the particle shape models used for computing the patterns shown in Figs. 7–10, respectively.

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Fig. 12.
Fig. 12. Left: patterns from Summit, width 5° (a)–(c); pattern (c) corresponds to plate diameter of ≈ 210   μm . Right: patterns computed from beam tracing for scalene plates with 3.6 and 36 μm edges, image width 16° (d), 9 and 91 μm edges, image width 10° (e), and a 100 μm diameter regular hexagonal plate, image width 10° (f), all 10 μm thick.

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Fig. 13.
Fig. 13. Left: as in Fig. 7; from the fringe spacing, the length of this column-like particle was estimated at ≈ 600   μm . Right: theoretical pattern for a 32 μm diameter, 160 μm long, hexagonal column with a prismatic facet tilted at 32° w.r.t. the incident direction, image width 20°.

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Fig. 14.
Fig. 14. Left: as in Fig. 7; the particle size estimated from the speckle [16] was 730 μm. Right: theoretical pattern for roughened hexagonal plate 40 μm diameter, 1.3 μm thick, incident angle 32°, image width 30°.

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